Servo mechanism



Nov. 16, 1948. a mm 2,454,107

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J T WRNEY Puma-anemic, 194's sEavo MECHANISM Sidney Wald, 'Collinnwood, N. 1., assignor to Radio Corporation of America, a corporation L Delaware Application December 31, 1947, Serial No. 794,978

This invention. relates to improvements in -,servo-mechanisms or follow-up systems, and

more particularly to methods and means for controlling the energization of an electric motor in response to deviations in the position of a load driven by said motor with respect to the. position of a control or input shaft.

It is ordinarily desirable in a follow-up system for the energization of the motor to be more or less smoothly or continuously variable between zero and some maximum value, the current at any given instant being a function of the deviation between the output shaft and the control shaft. This can be accomplished by supplying power to the motor through variable impedance devices such as resistors orthermionic tubes; however, such arrangements involve uneconomical use of the available power, sinceja large portion of it must be wasted in the impedance device. A further disadvantage lies in the fact that variable impedance devices for handlin substantial amounts of power must be large, heavy, and expensive; thermionic tube arrangements are usually complicated as well as bulky. Onofl' devices such as switches or relays may be used to control the motor, but these ordinarily do not give smooth stepless variation and may introduce hunting and inaccuracy of operation of the system.

It is the principal object of the present invention to provide an electrical follow-up system wherein the motor is controlled by switches or relays, but with a smoothness or continuity vof the same order as that provided by variable impedance devices.

' lay having variable length periods of closure alternating with substantially constant open circuit periods, and means for varying the frequency of operation to vary the average current supplied to the motor.

Still another object of the invention is to provide a system of the type set forth in the foregoing obiects which comprises a Wheatstone bridge circuit which is normally balanced to provide the same ratio between on and of! times i 2 Claims. (Cl. 318-28) 2 A f of the periodic switch the nominal position of the output shaft.

The invention will be described with reference 'to the accompanying drawing, wherein:

Figure 1 is a schematic diagram of a servo or follow-up system embodying the invention, and

Figures 2, 3 and rare oscillograms showing the operation of the system of Figure 1 under various conditions.

The system of Figure 1 includes a motor I- coupled as indicated'schematically by the dash line 3 to a load device, not shown. In the present example, the motor i is of the series wound D.-C. type, including an armature 5 and two field windings l and 9. The fields land 9 are so wound and connected that if the armature is energized through one, for example the winding 1, the motor tends to rotate clockwise, and if the armatureis energized through the other field,

the motor tends to rotate counterclockwise.

A source such as a battery II is provided for supplying energy to the motor I. and is connected to the terminal of the field winding 1 through the contacts i3 and i5 of a relay l1.' 'I'he'relay I! also includes an actuating coil I9, and a further contact 2|. when the coil I9 is energized less than a predetermined amount and the coil I9 is shortcircuited by engagement of the contacts l5 and 2| A capacitor 23 is shunted across the coil i 9.

The lower terminal of, the relay coil i9 is con- I nected to the source II; the upper terminal is connected through a fixed resistor 25 and a variable resistor 21 to one end of an adjustable voltage divider 29. The movable arm of the voltage divider 29 is returned to the source H, completing the circuit through the coil l9.

The terminal of the field winding 9 of the motor I is connected to the source ll through the contacts l3 and I5 of a relay H which is similar to the relay H. The relay ll is provided like the relay H, with a capacitor 23 across its coil 19', and is connected to the source I I through resistors 25' and 27' similar to the resistors25 and 21, and the voltage divider 29. The variable resistors 21 and 21' are mechanically ganged, as indicated schematically by the dash line 3|, and are driven by the motor i together with the load device. The connections are such that upon rotation of the motor so as to increase the resistance of one of the resistors 21 and 21, the resistance of the other will be decreased by a like amount.

The operation of the above described system is substantially as follows: Suppose the movable means independently of k The contacts l3 and I5 engage voltage applied to the coil 18 is the voltage between'the terminals of the capacitor 23. Initially, this is not sufllcient to operate the relay IT. The capacitor 23 continues to charge until its terminal voltage reaches the value necessary to cause the coil I9 to move the contact i into engagement with the contact 2!. when this occurs, the capacitor 23 and the coil iii are shortcircuited, and discharge rapidly until the coil I9 is no longer able to hold the contact II; in its upper position. The contacts l5 and I3 engage again, and the cycle is repeated.

During the periods when the contact I! is in its lower position and the capacitor 23 is charging, the motor I is fully energized through its field winding 1. During the periods when the contact I! is in its upper position and the capacitor 23 is discharging, the motor field 'l is disconnected. The lengths of the former periods (motor energized) depend upon the sum of the resistances 25, 21 and 29 included in the charging circuit or the capacitor 23. The lengths of the latter periods (field 1 disconnected) depend only upon the supply voltage, the capacitance of the capacitor 23 and the characteristics of the relay l1, and thus are constant and independent of the resistances of the resistors 25, 21 and 29.

Referring to Figure 2, with an intermediate value of total resistance in the circuit of the coil IS, the "on and oil periods are substantially equal. When thetotal resistance is higher, the "on periods become longer, as shown in Figure 3, while the oif periods remain the same length as before. Figure 4 shows the result of decreasing the total resistance in the circuit of the coil l9; the on periods become shorter, while the of? periods are still the same length.

The average voltage supplied to the motor i through its field winding 7 is proportional to:

i... t-i-k where t is the length of the on periods and it is the length of the ofi periods. The relay ill and its associated circuit elements are designed to make the periods 2? and is short enough, in the normal operation of the system, that the pulses of motor torque are averaged or integrated by the inertia of the motor armature and its load.

The relay i'l' operates like the relay H to periodically energize the motor i for rotation in the reverse direction. Preferably the oil time of the relay ii is of the same length it as that of the relay W. The average voltag applied to the motor tending to produce clocirwise rotation is then proportional to:

where t is the on time of the relay fill. When *2 and t are equal, the motor provides no average torque, and when t is greater than t, the average torque tends to produce counterclockwise rota= tion.

The motor i will run in the required direction,

increasing one oi'the resistances 21 and 21' and decreasing the other, until the "on periods t and t of the relays i1 and H are equal. Under this condition, the sum of the resistances 25, 21 and the upper portion of the voltage divider 29 is equal to the sum of the resistances 25', 21',

and the lower portion of the voltage divider 29. The motor l and its load will then remain stationary as long as no change is made.

The voltage divider 28 may be adjusted by a mechanical input, supplied for example by a manually operable crank 33. Motion of the crank 33 in either direction will add resistance in the circuit of one of the relay coils i9 and I9, and subtract an equal resistance from the circuit of the other relay coil. This increases the on" time of one relay and decreases the on time of the other; the motor! again drives the resistors 21 and 21' to equalize the on" periods t and t, and

then comes to rest. The position of the motor and its load thus corresponds substantially continuously to the position of the input device 33.

I claim as my invention:

1. A motor control system including an electric motor, means for energizing said motor including a periodic switch comprising a relay having an actuating coil and contact means shortcircuiting said coil upon energization of said coil by more than a predetermined amount, and further contact means which close to supply energy to said motor only when said first mentioned contact means are open; a capacitor connected in parallel with said actuating coil, and means including a variable resistor for charging said capacitor when said first mentioned contact means is open, at a rate depending upon the re-' sistance of said resistor, whereby the effective energization of said motor is controlled by said resistor.

2. A follow-up system' including an electric motor, means for energizing said motor to develop a torque in one direction of rotation, said means including a periodic switch having an actuating circuit, said switch remaining open during intervals of substantially constant lengths and remaining closed during intervals of lengths which depend upon the energization of said actuating circuit; further means, similar to said first mentioned means for energizing said motor to develop a torque in the opposite direction of rotation; a bridge network including the actuating circuit of said first mentioned means and that of said last mentioned means in two of its arms, and two variable resistors connected respectively in series with-said means; an adjustable voltage divider constituting two further arms of said bridge circuit, and means responsive to rotation of said motor to adjust said two variable resistors differentially, whereby said motor tends to maintain said bridge circuit in balance with the ratio between the lengths of the opened and closed periods of said first periodic switch equal to the ratio between the lengths of the opened and closed periods of the other of said periodic switches.

SIDNEY WALD.

No references cited. 

